(1) Field of the Invention
The present invention relates to an electrode structure and an electric heater comprising the electrode structure. More particularly, the present invention relates to an electrode structure provided in an exhaust gas system of an internal combustion engine and connected to, for example, a conductive substance accommodated in a metallic housing in order to apply electricity to the conductive substance to heat it and thereby purify the exhaust gas generated during the cold periods (e.g. cold start) of engine, as well as to an electric heater comprising the electrode structure.
(2) Description of the Prior Art
A technique is known in which, in a catalytic converter used for purification of the exhaust gas generated in automobiles, etc., an electric heater is provided to raise the temperature of the converter catalyst to its working temperature as quickly as possible.
In this technique, as shown in FIG. 20, an electric heater 101 and an auxiliary catalyst 102 are provided upstream of a main catalyst 100, that is, between an engine and the main catalyst 100 in an exhaust pipe 150, to purify harmful substances present in the exhaust gas generated during cold engine periods. In this system for exhaust gas purification, when the main catalyst 100 is not activated (for example, during cold engine periods such as cold start and the like), the electric heater 101 is put in an ON operation for a given length of time (e.g. 40 seconds) to raise the temperatures of the electric heater 101 and the auxiliary catalyst 102 to respective given temperatures (e.g. 400-500.degree. C.) or above to activate the electric heater 101 and the auxiliary catalyst 102 in a short time and allow the electric heater 101 and the auxiliary heater 102 to conduct gas purification during cold engine periods. After the engine has been warmed up, purification of harmful substances present in the exhaust gas is conducted by the electric heater 101, the auxiliary heater 102 and the main catalyst 100.
The above electric heater generally comprises: a metallic honeycomb structure; a metallic housing (e.g. a can) for accommodating and holding the honeycomb structure; and an electrode for applying electricity to the honeycomb structure which is a conductive substance.
As electrode structures of electric heaters, there have conventionally been known those shown in FIGS. 21(A)-21(C).
In FIG. 21(A), a positive screw portion 103 is provided by carving. An electrode pole 105 Is fixed by an electrode 104. A first and a second insulating members 106 and 107, respectively, are engaged with the electrode pole 105 around the electrode pole 105. In a gap between the first insulating member 106 and the second insulating member 107 is placed a ceramic powder 108 such as cement. The ceramic powder 108 is packed in a space formed by the first and the second insulating members 106 and 107 and a fixing ring 109. On the electrode pole 105, a nut 111 is thread-mounted via a washer 110. Thus, the first and the second insulating members 106 and 107, and the like are fixed to the electrode pole 105.
In the conventional embodiment shown in FIG. 21(A), an electrical insulation against a metallic housing (case) 160, which works as an exhaust pipe, is ensured by the first and the second insulating members 106 and 107. An exhaust gas is sealed by the ceramic powder 108. The electrode 104 inside the metal housing 160 is connected with a part of honeycomb structure 133 by means of a metallic connecting member 135.
FIG. 21(B) shows another electrode structure called a plasma coating type. An insulating member 112 such as ceramic is coated on a predetermined portion of a pole by a discharge of plasma so as to obtain an electrode pole 113. The electrode pole 113 is inserted in and engaged with a ring 115 fixed to a metallic housing 114.
In this plasma coating type of electrode structure, the aforementioned coated portion having the insulating member 112 is forcibly inserted in and engaged with the ring 115, by which an electrical insulation against a metallic housing and a sealing against an exhaust gas are ensured.
FIG. 21(C) shows still another electrode structure called a metal seal type. The top end portion of an electrode pole 117 is provided with a positive screw portion 116 by curving. The electrode pole 119 is fixed to an electrode 117. A first insulating member 120 and a second insulating member 122 are engaged with the electrode pole 117 therearound. The first insulating member 120 is provided with a first ring 118 and a second ring 127 on both the upper and the lower surfaces of a protrusion. That is, an electrode structure of a metal seal type is provided with the first insulating member 120 and the second insulating member 122 are fixed in predetermined places by a net 123 and inserted in and engaged with a metallic housing 126.
The metal seal type of electrode structure is electrically insulated against a metal housing 126 by the first and the second insulating members 120 and 122 and sealed against exhaust gas by the first and the second metallic ring 118 and 127.
In the electric heater comprising such an electrode structure, there is preferred, for conveniences such as easy fitting to an exhaust pipe of an automobile, etc., an electrode structure which is connected to a lead wire connecting to an external electric source and in which one end of the lead wire is contained for reliable electrical connection between the electrode and the lead wire. This electrode structure, however, has problems in that, when an automobile is driven in rainy weather, etc. on rain puddles of roads or during flooding, water adheres onto the electrode of the electric heater 101, which may cause electrical short-circuiting and consequent misoperation of heater (see FIG. 22).